The invention relates to an improved process for applying a uniform wet finish coat to woody substrate and particularly to non-uniform or textured hardboard.
Wood based fiber and particle panel materials include insulation boards, medium-density fiberboards, hardboards, particle-boards, and laminated paperboards. Hardboard and medium density fiberboard is manufactured from wood and other ligno-cellulosic materials they have been chemically converted to fibers and then reformed into panels by the application of intense heat and pressure. The heat and pressure cause the lignin in the wood to soften and form a bond, creating a dense, grainless material that has high strength and water resistance. Binders and other additives may be added prior to the pressing step.
Hardboard is pressed with a smooth surface or with a non-uniform (textured) surface. Textured hardboard is designed to simulate the irregularities of weathered wood, brick, masonry, etc. It is difficult to apply a uniform coat of paint over these textured surfaces at the low viscosity needed for coating application, i.e., in the order of from about 9 to about 15 seconds on a number 3 Zahn cup. When such low viscosity aqueous coatings are applied to the surface to textured boards the coatings run down into the low areas of the texture; it also penetrates the low density areas of the ridges and leaves these high (ridge) areas uncoated or unevenly coated. This occurs even when the coating does not exhibit sag.
Prior art techniques of adding hold-out agents and/or thickening agents to the coatings have not solved the problem. This indicates that the mere change of aqueous coating viscosity is not the solution and can actually be detrimental in producing over or premature gellation and thus causing stoppage and down-time losses.
Several attempts have been made to overcome this problem with limited success: Using a direct roll coater a very heavy film is deposited in the low areas at the ridge base while a low ridge coverage is due to the roll pressure; several applications are needed. In spray applications the paint strikes into the low density areas leaving a non-uniform surface coating. Curtain coating techniques leave uncovered the area behind the ridges, in the direction of travel. With brush coaters low areas accumulate puddles of heavy coating not removed by the brushes.
Various combinations of the above application techniques have been attempted but still fail to provide satisfactory film coverage. More recently the "pneumatic" coater technique has been applied to this operation. In this process a heavy wet film is flooded over the surface and subsequently an air-knife strips excess coating from the surface. The air knife is applied about 0.5-1.0 second following the flooding procedure. Although the air knife/flooding technique gives some improvement, coating uniformity was not acceptable even by preheating the substrate to 150.degree. to 180.degree. F. The present invention represents an improved process for coating uneven and irregular wood or hardboard surface by closely controlling two parameters namely (a) preheat temperature of the substrate and (b) contact time between flooding the surface and stripping excess with an air knife.